Differences among Monte Carlo codes in the calculations of voxel values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

Abstract

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel S values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel S values for monoenergetic sources (electrons and photons) and different radionuclides (90Y, 131I, and 188Re) were calculated. Simulations were performed in soft tissue. Three general-purpose MC codes were employed for simulating radiation transport: MCNP4C, EGSnrc, and GEANT4. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the EGS4 MC code, were also included in the comparisons. The impact of the differences (in terms of voxel S values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self-irradiation voxel S values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel S values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel S values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for 90Y the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel S values) with respect to the EGS4 code. Further, variations were observed up to about 30%, for small source-target voxel distances, when low-energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for 131I). For 90Y and 188Re, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel S values for radionuclide targeted therapy dosimetry. However, if a low-energy beta-emitting radionuclide is considered, these differences can affect also dose depositions at small source-target voxel distances, leading to more conspicuous variations (about 9% for 1311) when calculating the absorbed dose in the volume of interest.

Impact of SPECT corrections on 3D‐dosimetry for liver transarterial radioembolization using the patient relative calibration methodology

“…16 An optimal treatment planning should include 3D voxelbased dosimetry, accounting for nonuniform absorbed dose distributions when studying dose-effect correlations. 9,[17][18][19][20][21][22][23] Image-based 3D dosimetry can be performed in several ways: direct Monte Carlo (MC) simulation, which is considered the gold standard; 9,[24][25][26][27][28][29][30] convolution calculations by voxel S-values, reliable in nearly uniform density tissue; 19,[31][32][33][34][35][36] local energy deposition method. 18,20,[36][37][38][39] Activity distribution quantification by SPECT is the major concern, due to physical and clinical degrading factors of the images.…”

A review of 3D image-based dosimetry, technical considerations and emerging perspectives in 90Y microsphere therapy

“…A comparison of MC codes (MCNP4C, EGSnrc and GEANT4) used to define the S-value matrix for β-emitters has been shown to affect dose distribution values by only a few percent [93]. MIRD pamphlet 17 originally produced VSVs for 3 voxel sizes and 5 isotopes although more recent work expanded these tables using simulations within the EGSnrc toolkit (and validated with PENELOPE and MCNP4c codes) with more isotopes and a larger range of voxel sizes more suited to modern scanners [94].…”

The COOLER Code: A Novel Analytical Approach to Calculate Subcellular Energy Deposition by Internal Electron Emitters

“…Because detailed simulation of electron tracks can be time consuming, condensed history transport codes have often been employed to approach various cellular dosimetric problems (26)(27)(28)(29)(30)(31)(32). Among the shortcomings of using condensed history codes, we point out the adoption of large energy cutoff for electron transport, typically between 1 and 10 keV, which results in a spatial resolution of the order of the biological target.…”